Aqueous nitrogen-based fertilizers are widely used in agriculture because of their ease of application and effectiveness. The corrosivity of such fertilizer solutions has been a major problem associated with the handling of these materials. Such fertilizer solutions cause severe corrosion of liquid fertilizer handling system components, such as metal mixing and storage vessels, pumps, valves and pipes. The need for special precautions in the handling of aqueous nitrogenous fertilizer solutions and the high cost of special materials such as stainless steel vessels and lines adds considerably to the expense of storing and transporting these products. The term "nitrogenous fertilizer" is used herein to mean nitrogen-containing compounds that have a nutritive effect on plants. Nitrogenous fertilizers include ammonia; ammonium salts, such as ammonium nitrate, ammonium sulfate, ammonium phosphate, etc.; nitrates, such as potassium nitrate, sodium nitrate, etc.; organic nitrogen-based compounds, such as urea, ammonium carbamate, etc.; and mixtures of these. Aqueous solutions of these compounds generally have a highly corrosive effect on various metals, particularly ferrous metals and aluminum.
Many attempts have been made to eliminate or at least reduce the corrosive effect of nitrogenous fertilizer solutions on metal parts and equipment. Various chemical treatments have been tried with varying degrees of success. For example, U.S. Pat. No. 2,680,766 teaches that corrosion of chromium steel alloy equipment that is used in the manufacture of urea from carbon dioxide which contains small amounts of sulfur can be reduced by introducing small amounts of copper to the system. This patent also states that the amount of oxygen in the reactants should be less than 10 parts per million parts of reactants (ppm). U.S. Pat. No. 3,230,070 teaches the use of cupric amines to inhibit the corrosive characteristics of aqueous solutions of nitrogenous materials. U.S. Pat. No. 3,466,192 teaches contacting metal surfaces with soluble methylol thiourea compounds an oxidizing acid solution. Included in the oxidizing acid systems disclosed in this patent are mixtures of hydrogen peroxide and hydrofluoric acid or hydrochloric and nitric acids. U.S. Pat. No. 3,720,548 discloses the use of small amounts of an oxidizing agent, such as oxygen, air or hydrogen peroxide to render austenitic stainless steels less susceptible to corrosive attack by aqueous ammonium carbamate and urea solutions. The metal is first exposed to the aqueous carbamate solution and then it is contacted with the oxidizing agent. Several patents teach the introduction of air or oxygen to a reactor in which urea is made to inhibit corrosion caused by the reactants or products. For example U.S. Pat. Nos. 2,727,069, 3,137,724 and 3,574,738 teach inhibiting the corrosive effects of ammonium carbamate and urea solutions on metal surfaces of reactors used to synthesize these compounds by introducing small amounts of oxygen into the reactors during the synthesis. U.S. Pat. No. 2,959,556 teaches that metal surfaces can be protected against the corrosive effects of halogenated hydrocarbons and their hydrocarbon solutions by adding to these liquids small amounts of organic peroxides.
The above patents disclose techniques which improve the protection of metal surfaces against the corrosive effects of nitrogenous materials. However aqueous nitrogenous solutions are so corrosive that new and improved methods for protecting metal surface from their corrosive effects are continuously being sought.